1. Field of the Invention
The present invention relates to an image reading apparatus that reads an image printed on an original document, and an image forming apparatus provided with the image reading apparatus.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application, First Publication No. 2004-320331 discloses a technique for an image reading apparatus that reads an image printed on an original document, for preventing “show-through” (a phenomenon, in which an image on the back surface appears in an image on the front surface) that occurs when reading an original document with printing on both sides. The image reading apparatus disclosed on this patent document is provided with: an image reading unit including a light source, a mirror, a lens, and a CCD (charge coupled device) element; and a transparent platen roller that is arranged so as to oppose to the image reading unit and clamp the original document, and that is provided on the inside with a cylinder-shaped background plate sectioned along the circumferential direction into two areas, namely a white area and a black area. In this image reading apparatus, when reading an original document with printing on one side, the transparent platen roller is rotated so that the white area thereof faces the image reading unit side, and on the other hand, when reading an original document with printing on both sides, the transparent platen roller is rotated so that the black area faces the image reading unit side. According to the above technique, an original document with printing on one side can be read with an excellent level of contrast, and an original document with printing on both sides can be read with no show-through.
Incidentally, as image reading methods used in an image reading apparatus, as disclosed in the above patent document, there are widely known: the CCD method in which among the light irradiated from the light source onto an original document, reflected light from the original document is received on a CCD element via an optical system including a mirror and a lens; and the CIS method in which reflected light acquired by irradiating light of three colors, namely red, green and blue, from a LED light source onto an original document, is received on a CIS (contact image sensor) element.
In an image reading apparatus that employs the conventional CIS method, as shown in FIG. 5, a CIS unit 100 (image sensor unit including a LED light source 100a and a CIS element 100b) and an original document transfer roller 200, which also serves as a white reference member for shading correction, are arranged opposed to each other. An original document P set in a paper feed tray is automatically transferred between the CIS unit 100 and the original document transfer roller 200, and an image printed on the original document P is thereby read.
In an image reading apparatus of this type, there has been problem in that when reading an image on the front surface of the original document P with printing on both sides, as shown in FIG. 5, the illumination light of the LED light source 100a transmitted through the original document P is reflected on the original document transfer roller 200 and enters the CIS element 100b, resulting in an image on the back surface of the original document P appearing in the image on the front surface.
In the case where the technique of the above patent document is employed in order to solve this kind of problem, in the original document transfer roller 200 there is provided a cylinder-shaped background plate sectioned along the circumferential direction thereof into two areas, namely a white area and a black area. However, the original document transfer roller 200 serves as a white reference member for shading correction, while also serving as a roller for transferring original documents. Therefore, there is a need for performing complex control to rotate the original document transfer roller 200 to transfer the original document P, and so that when correcting shading, the white area faces the CIS unit 100 side, and when reading the image, the black area faces the CIS unit 100 side. Furthermore, during transferring of the original document P, the original document transfer roller 200 must be rotated so that the white area does not face the CIS unit 100 side. Therefore, it is necessary to make the diameter of the original document transfer roller 200 greater (the circumferential direction length needs to be made longer). It is structurally difficult to actually realize such an image reading apparatus (due to an increase in the scale of the apparatus).
The present invention takes the above circumstances into consideration with an object of providing an image reading apparatus that is capable, with a simple control and structure, of suppressing the occurrence of the show-through phenomenon, and an image forming apparatus provided with the image reading apparatus.